The former is an opinion, the latter is based on the fact that Congress holds the purse strings. He has 90 days to withdraw or he needs the approval of Congress and he must give Congress 48 hours prior warning...
Sadly the POTUS (not just current incumbent) believes he has the power without reference to Congress irrespective of what Congress wants.
RR declared was on Nicuragua by using proxies including supplying mines to be used against it but never got Congressional approval for it, nor the Iran weapon debacle.